This Application is a U.S. National Phase Application of PCT International Application PCT/JP99/02206 filed Apr. 26, 1999.
1. Technical Field
The present invention relates to an oscillator for use in portable telephones and the like.
2. Background Art
Generally, this type of oscillator used to comprise an oscillating element and a first amplifier connected to input and output terminals of the oscillating element. In order to attain a small size and a low cost, the first amplifier used to comprise a first P-type transistor and a first N-type transistor both fabricated by MOS process. In other words, a gate of the first P-type transistor and a gate of the first N-type transistor of the first amplifier are connected to the output terminal of the oscillating element. And a drain of the first P-type transistor and a drain of the first N-type transistor are connected to the input terminal of the oscillating element. Furthermore, a source of the first P-type transistor is connected to a power supply terminal and a source of the first N-type transistor is connected to a grounding terminal. Also, the gate of the first P-type transistor is configured in a manner such that it is spanned between the P-type source diffused region and the P-type drain diffused region while the gate of the first N-type transistor is configured in a manner such that it is spanned between the N-type source diffused region and the N-type drain diffused region.
In the above configurations, an output from the oscillating element is applied to the gates of the first P-type transistor and the first N-type transistor. By the application of an output voltage of the oscillating element to these gates, an electric field is produced in the bottom portion of the gate between each respective source diffused region and drain diffused region. As a result of this, a conductive channel is created between the source diffused region and the drain diffused region over which the gate has been spanned. Application of an alternating voltage to the gates causes the first P-type transistor and the first N-type transistor to become alternately conductive thereby amplifying the oscillation output and feeding it back to the input terminal of the oscillating element. This feedback keeps the oscillation continuing.
When the first amplifier is configured with the first P-type transistor and the first N-type transistor fabricated by MOS process, a small and low cost oscillator can be configured. However, employment of such MOS process suffered a problem of an increase in the noise as many frequency components deviated from the desired oscillation frequency were generated below and above the desired oscillation frequency.
While the reason for this has not been fully investigated, the following mechanism may be considered. Namely, when voltage is applied to the gates and the corresponding parts, an electric field is produced leading to the creation of a conductive channel. Because of non-uniformity and the like of the structural compositions of the portion between the drain and the source, the conductive channel is disturbed and noise is produced by the disturbance.
An object of the present invention is to reduce the level of noise other than the desired oscillation frequency.
In order to attain the object, the present invention employs a configuration in which the gate length of the first N-type transistor is made longer than the gate length of the first P-type transistor.
In oscillators in general, when an N-type transistor and a P-type transistor are compared, the gate length of the N-type transistor is made shorter than the gate length of the P-type transistor in order to equalize the gain. As a result, a strong concentrated electric field is produced in the portion under the gate and between the drain and the source of the N-type transistor. In the present invention, however, the electric field intensity can be reduced by making the gate length of the N-type transistor longer than the gate length of the P-type transistor. Accordingly, the present invention can reduce the level of noise other than the oscillation frequency.